The present invention relates to electrical ignition systems for internal combustion engines and, more specifically, to a contactless ignition system utilizing a saturable core transformer.
Recently, significant effort has been directed to the development of a contactless ignition system for internal combustion engines. The contact ignition system has certain disadvantages which may best be understood by reviewing briefly the basic operation of the conventional ignition system which utilizes contacts. In the conventional system, the vehicle DC power source, such as the battery or generator, provides a DC current to a primary circuit comprising the series connection of the ignition switch, the primary coil of an ignition coil assembly and a pair of contacts which are opened and closed in response to the rotation of the distributor shaft. The primary coil is magnetically coupled to a secondary coil which has a greater number of turns than those in the primary coil. Current flows in the primary circuit when the distributor contacts are closed. This current causes a magnetic field to build up in the primary coil. When the distributor contacts open, the current in the primary circuit tends to continue to flow. A capacitor which is connected across the distributor contacts provides a reservoir for this current, and reduces the tendency for the current to arc across the open distributor contacts. When the current decreases, the magnetic field decreases, and this sudden change in the magnetic field induces a high voltage into the secondary coil of the ignition coil assembly. This voltage is sequentially directed across each of the sparkplugs resulting in an arc which ignites the mixture of gasoline and air in the cylinder.
There are two basic reasons why contact ignition systems require periodic maintenance. The first reason is contact deterioration which is a result of the inability to prevent some arcing from occuring across the contacts. The second is the wearing down of the rubbing block which results in changes in the contact or dwell angle and in the spark timing. The rubbing block which is affixed to the distributor shaft is the cam which opens and closes the contacts. Thus, the use of distributor contacts is the primary reason for frequent ignition system maintenance.
Several contactless ignition systems have been developed, but none have been developed which do not introduce additional problems. One common problem with these systems is their inability to operate as effectively at slow cranking speeds as they do at higher speeds.
One of the first systems in the direction of the semi-conductor ignition system did not actually eliminate the contacts, but instead, it reduced the current flowing through them. The contacts were connected in series with the base circuit of a transistor, the collector and terminal electrodes being connected in series with the primary coil. While this system reduced the current through the contacts by placing the contacts in a relatively low current transistor base circuit, it did not solve the problem of rubbing block wear and deterioration. In another known circuit, the contacts were replaced by a magnetic assembly which was mounted on the rotor of the distributor and a pulse amplifier network. The assembly included a number of permanent magnets which were evenly spaced around the rotor and a group of stationary pickup windings. The rotating magnets altered the magnetic field which is coupled to the stationary windings to provide pulses to the amplifier in synchronism to the crankshaft rotation. The amplifier output is applied to the low voltage winding of the ignition coil. While this system eliminates the contacts and their attendant problems, this system does not operate effectively at low cranking speeds since the size of the pulse is related to the speed of the rotor; as the pulse becomes smaller, noise in the system has an increased disadvantageous effect.
The Huntzinger U.S. Pat. No. 3,357,416 addressed its teachings to this problem which is stated in the patent to be as follows:
"The potential pulses produced by a magnetic pulse generator of this type are of a low amplitude at the slow rotational breaker cam speeds encountered during cranking, thereby necessitating a large amplification of the developed signal. The large amplification required renders the system extremely sensitive to spurious signals, produced by mechanical vibration and electrical transient impulses present in the power system, which may create ignition signals at unwanted times."
The Huntzinger patent discloses an ignition system including a magnetic pulse generator in which the flux coupling a pickup coil relates to the position of the distributor rotor, an electronic integrator and amplifier for integrating the pulse to reduce the effect of spurious noise, a multivibrator driven by the integrator output and a normally conducting transistor whose collector and emitter electrodes are in series with the primary coil, the transistor base being driven by the multivibrator. The current in the coil is interrupted when the magnetic pulse occurs. While this circuit does reduce the electrical noise problems, this system consumes significant power and is not economical to manufacture due to its relatively complicated circuit.
Another system is described in U.S. Pat. No. 3,407,795 to Aiken et al. In this patent, a Jenson oscillator provides an AC signal which drives a transformer, the output of which is applied to the proper sparkplug. Another transformer provides a feedback signal for the oscillator. A set of conventional contacts controls the current through one winding of the feedback transformer such that the feedback transformer is saturated before the time for firing. When the points open, the oscillator starts, thereby providing an AC signal to the sparkplug. Another network which includes a silicon controlled rectifier provides an alternate path for the current for the saturating winding of the feedback transformer to stop the AC voltage to the sparkplug at a time after the opening of the contacts, the time being independent of the engine speed. This system, while teaching the use of an oscillator, utilizes conventional contacts. The magnetic field for saturation of the feedback transformer is created by an electrical current flowing through a winding. This current creates an added power drain and may deteriorate the contacts since this current flows through them.
Various other ignition systems, and magnetic pickup devices have been described in the prior art, such as U.S. Pat. Nos. 3,434,463, 3,575,150, 3,675,635, 3,500,809, 3,484,677 and 3,658,039. However, the prior art of which the inventor is aware neither discloses nor suggests a contactless ignition system of the type described and claimed in the present invention.